Angiographic estimation of atherosclerotic tissue thickness is important in the determination of atherosclerotic regression or progression in serial angiographic studies. This requires the prediction of non-atherosclerotic arterial taper. In this study, normal arterial taper has been characterized by measurements on angiograms. A formula derived from the principle of minimum work has been found predictive of the change in arterial width due to branching on in vivo angiograms (rabbit aortas) and in postmortem angiograms (rabbit aortas and human coronary arteries). This formula is now being used to predict proximal atherosclerotic tissue thickness from distal normal segments and normal branch diameters. The difference between the atherosclerotic lumen (by angiogram) and the predicted non-atherosclerotic taper represents atherosclerotic tissue thickness. Observations of human coronary casts have demonstrated the feasibility of this approach. This measure of atherosclerotic thickness is being tested in vivo during atherosclerotic progression and regression in rabbits with serial angiography. The angiographic measures are being applied to the estimation of human coronary atherosclerotic tissue thickness by evaluation of postmortem angiograms. Angiographic predictions will be confirmed by histological measurements of atherosclerotic tissue thickness. This method will be valuable in assessing atherosclerotic regression or progression by serial angiography in tapering arteries.